In silicon integrated circuit (IC) fabrication, it is often necessary to isolate semiconductor devices formed in the substrate. This is true for many semiconductor memory devices, for example, DRAM, flash memory, SRAM, microprocessors, DSP and ASIC. The individual pixels of a CMOS image sensor also need to be isolated from each other.
A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells including a photogate, photoconductor, or photodiode overlying a charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a floating diffusion node and a transistor, for resetting the diffusion node to a predetermined charge level prior to charge transference. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the diffusion node and an access transistor for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the floating diffusion node. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by a source follower output transistor. The photosensitive element of a CMOS image sensor pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate. A photon impinging on a particular pixel of a photosensitive device may diffuse to an adjacent pixel, resulting in detection of the photon by the wrong pixel, i.e. cross-talk. Therefore, CMOS image sensor pixels must be isolated from one another to avoid pixel cross talk. In the case of CMOS image sensors, which are intentionally fabricated to be sensitive to light, it is advantageous to provide both electrical and optical isolation between pixels.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256.times.256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046–2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452–453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe operation of conventional CMOS image sensors, the contents of which are incorporated herein by reference.
Shallow trench isolation (STI) is one technique, which can be used to isolate pixels, devices or circuitry from one another. In general, a trench is etched into the substrate and filled with a dielectric to provide a physical and electrical barrier between adjacent pixels, devices, or circuitry. Refilled trench structures, for example, are formed by etching a trench by a dry anisotropic or other etching process and then filling it with a dielectric such as a chemical vapor deposited (CVD) silicon dioxide (SiO2). The filled trench is then planarized by an etch-back process so that the dielectric remains only in the trench and its top surface remains level with that of the silicon substrate. The depth of a shallow trench is generally from about 2000 to about 2500 Angstroms.
One drawback associated with shallow trench isolation in the case of CMOS image sensors is cross-talk from a photon impinging on a particular pixel of a photosensitive device causing changes that may diffuse under the shallow trench isolation structure to an adjacent pixel. Another drawback is that a hole accumulation layer along the sidewall of the trench is relatively small since it is limited by the depth of the shallow trenches.
One technique which may be used to improve pixel isolation in CMOS image sensors is to implant dopants beneath the isolation region; however, it has been found that this may contribute undesirably to pixel dark current. Minimizing dark current in the photodiode is a key device optimization step in CMOS image sensor fabrication.
It is desirable to provide an isolation technique that prevents cross-talk between pixels while reducing dark current or current leakage as much as possible. It is also desirable to provide an isolation technique while increasing a hole accumulation region adjacent a pixel isolation region.